Quota Implementation

[Arne Jansen <sensille@gmx.net>]

Hi,

If no one is already working on it, I'd like to take the Quota lock and
see how far I come.
Let me sketch out in short what I'm planning to do:

  - Quota will be subvolume based. Only the FS-trees and data extents
    will be accounted.
  - Quota Groups can be defined. Every quota group can comprise any
    number of subvolumes. A subvolume can be assigned to any number
    of quota groups.
  - A Quota Group can account/limit the total amount of space that is
    referenced by it and/or the amount of space that is exclusively
    referenced (i.e. referenced by no other quota group).
  - With this it is possible to define a hierarchical quota that need
    not necessarily reflect the filesystem hierarchy.
  - It is also possible to decide for each snapshot if it should be
    accounted into the parent group. So in a scenario where each
    subvolume reflect a user home, it's possible to have some snapshots
    accounted to the user and others not (e.g. the ones needed for system
    backups).
  - Quota information will be stored in new records, possibly in a
    separate tree.
  - It should be possible to change the Quota config and group
    assignments online, though this might need a full re-scan of the fs.
  - It does NOT include any kind of user/group (UID/GID) quota.

Any addenda or arguments why it's impossible or insane welcome.

-Arne